Magnetron tube with cavity resonator



Nov. 3, 1953 J. E. EVANS ETAL U MAGNEITRON TUBE WITH CAVITY RESONATORFiled March 1, 1946 INVENTORS JOHN E. EVANS ROBERT c. FLETCHER FOSTER F.RIEKE ATTORNEY Patented Nov. 3, 1953 MAGNETRON TUBE WITH CAVITYRESONATOR John E. Evans, Houston, Tex., Robert C. Fletcher,

- Cambridge, and Foster F. Ricks, Belmont, Mass., assignors, by mesneassignments, to the United States of America as represented by theSecretary of War Application March 1, 1946, Serial No. 651,300

This invention relates in general to cavity type 3 Claims. (Cl. 315-39)magnetrons, and more particularly to means for coupling radio frequencyenergy from such devices.

As an oscillator for the generation of radio frequencies havingassociated wavelengths in the centimeter range, the cavity magnetron isvery satisfactory. One form of this thermionic tube has a linearcathode, and an anode structure consisting of a number of cavityresonators positioned symmetrically about the cathode. This structuremay take any one of a number of different shapes, the various formsusually being the result of the number of designs of the cavityresonators that are possible. These designs are often dictated by thedesirable ease of manufacture of the tube. One such structure isincorporated in the so-called vane type magnetron, in which the cavityresonators become the volume between flat vanes which are positioned soas to flare out radially from the cathode-anode space.

A cavity magnetron is quite sensitive to loading conditions, and a smallchange in the amount of loading will usually cause the tube to oscillateat a slightly different frequency. This phenomenon of frequencyvariation with loading is termed pulling. The tighter the couplingbetween magnetron and load, the greater the frequency variation.

Some types of cavity magnetrons utilize a loading arrangement coupled tothe output line associated therewith as a means of frequency pulling. Asa result of some undesirable features of this arrangement, thedouble-output" cavity magnetron was developed. This tube has a tuningstructure tightly coupled to one cavity thereof, and the regular outputtransmission line is loosely coupled to another cavity, and thevariation in the loading conditions of the regular output thus has arelatively small effect on the tube operating characteristics. Theabove-mentioned tuning apparatus usually takes the form of a tunablecavity resonator of some sort coupled to the magnetron. If a largefrequency range is desired, not only must the cavity have the necessarytuning range but the coupling between the tuning cavity and themagnetron must be able to pass frequencies over the entire band atnearly the same level; that is, the coupling must have a low eifectiveQ. An eifective Q less than about may be considered to be a low Q.

Accordingly, among the objects of the present invention are:

i. To provide a double output cavity magne= tron;

2. To provide a means of coupling to such a magnetron whereby arelatively large range of frequencies may be obtained; and

8. To obtain such a frequency range by providing a coupling means havinga relatively low effective Q.

This invention will best be understood by reference to the drawing, inwhich Fig. l is a sectional view of a portion of a double-output vanetype magnetron employing one embodiment of the principles of the presentinvention, and Fig. 2 shows a sectional view taken along the line 2-2 ofFig. 1. It will be understood that details not necessary to theunderstanding of the present invention are omitted from the drawing forpurposes of simplicity.

Referring now to a description of the apparatus and to the drawing,there is represented a cathode l0, positioned symmetrically about whichare radially extending anode vanes H, l2, l3, l4, I5, [6, i1 and I8.Surrounding the anode vanes and firmly attached to them is a shell IS.The view shown in Fig. 1 of the drawing is one looking in a directionparallel to the long straight cathode of the tube, and therefore theplane of the sheet of drawing is normal to the axis of the cathode. Thevanes, cathode and shell extend in a direction normal to the plane ofthe paper by substantially equal amounts, the shell overlapping theother elements by a small amount in this dimension. The dimensions ofthe vanes, cathode, and shell in planes parallel to the plane of thedrawing are constant for a short distance in a direction normal to theplane of the drawing as is well known to the art. To complete the tubesmain structure, end plates 40, 4| (Fig. 2) are fitted on both open endsof the tube. These plates would be parallel to the plane of Fig. 1 ofthe drawing. Thus the vanes and the shell form cavity resonators 20, 2!,22, 23, 24, 25, 26 and 2'6 which are dispersed symmetrically about thecathode.

The tuning structure consists of a coaxial cavity resonator 28 having anouter conductor 28, an inner conductor, and a movable plunger 3|. The 11arrows show the directions of motion of the plunger. In order to couplethe coaxial cavity to the magnetron, the inner conductor 30 is extendedinto the magnetron cavity 2! and formed into the partial loop 32. Member32 is brought into the cavity 2i parallel to, and very close to, onewall oi vane H. When the neighborhood of the inner end of the vane isreached,

member 32 bends over and is soldered to the end of vane 03.

The output power is coupled from the magnetron by means of coaxial line34, comprising inner conductor 35 and outer conductor 38. The innerconductor is brought into magnetron cavity 25, bent into the form of aloop 31, and soldered to the back wall the cavity, which isa portion ofthe shell l9.

Referring now to a description of the operation or the apparatus and tothe drawing, it may be seen that the combination or member 32 and vaneIt forms an effective loop which is linked by the magnetic field withinthe cavity. The large area of this eflective loop provides a relativelyhigh coupling from the magnetron to the coaxial tuning cavity 2|.because the loop is made to embrace a relatively large amount of fluxwithin the cavity II.

The magnetron is affected by the coaxial cavity 28 being stronglycoupled to it only when the magnetron is generating frequencies in theneighborhood of the frequency to which the cavity 2! is tuned. Theresonant ireuuency of cavity 28 then may be varied by changing theposition oi plunger 3! and therefore the volume of the cavity into whichoscillating energy is coupled. Elements such as used in this type oi.loop coupling are inherently irequency sensitive in the centimeterrange, because their dimensions are of the order of a wavelength at theoperating frequency. The effective Q of a device such as this loopdepends to a large extent on the closeness of member 32 to the wall ofvane i4. Thus it is seen that an effective two-conductor transmissionline is formed over a short distance, with the straight portion ofmember 32 being one conductor and the flat vane ill being the otherconductor. From transmission line theory it may be said that the spacingof these two conductors determines the characteristic impedance of thisline. It is experimentally found that this spacing also affects the Q ofthe coupling arrangement.

Using a relatively low voltage magnetron oi. the

"2J39" type which has been manufactured by the Raytheon ManufacturingCompany, a 0.062" wire as used for member 32 gave a loaded Q oi 3.5 fora spacing of 0.005" between member 321 and vane ll, and a loaded Q of9.6 for a spacing of 0.010. Tuning ranges of about iii% of the centerfrequency have been obtained using this type of loop.

The oscillating energy is coupled out in the normal manner by magneticcoupling ioop ti.

While there been iiiescrlhed "What is present considered the preierredembodiment oi the invention, it will be obvious to those skilled in theart that various changes and modifications may be made therein withoutdeparting ironi the invention and it is, thereiore, ainicd in theappended claims to cover all such changes and modifications asialLu/ithin the true spirit and scope of the invention.

What is claimed is:

1. A double-output vane-type cavity magnetron, a coaxial cavityresonator including an inner conductor and an outer conductor, meanscoupling said coaxial cavity resonator to a first cavity of saidmagnetron comprising an extension 01 said inner conductor into saidfirst cavity, a portion or the length of said extension being positionedsubstantially parallel and very close to a first vane of said magnetron,and the end of said inner conductor being fastened to a second vane 01said magnetron at the end near the center of said magnetron or saidsecond vane. said first vane being positioned adjacent to said secondvane, and output means coupled to a second cavity of said magnetron.

2. The combination of a double-output vanetype cavity magnetron, acoaxial cavity resonator i'or tuning said magnetron including an innerconductor and an outer conductor, said cavity resonator having aconductive closure at one end, means coupling said coaxial cavityresonator at its other end to a first cavity of said magnetroncomprising an extension of. said inner conductor, a portion of thelength or said extension being positioned close and substantiallyparallel to a first vane of said magnetromthe end of said irmerconductor being fastened to a second vane of said magnetron at a pointnear the inner end of said second vane, said second vane being adjacentto said first vane, and output means coupled to a second cavity of saidmagnetron.

3. A double-output vane-type cavity magnetron, and a cavity resonatorhaving a low effective Q comprising a coaxial line for tuning saidmagnetron including an inner conductor, an outer conductor, and aconductive plunger in said coaxial line for effectively short-circuitingsaid coaxial line, said inner conductor being extended into one of thecavities of said cavity magnetron, a portion of the length of theextension being positioned substantially parallel to and very close to afirst vane of said magnetron, the end oi said inner conductor beingfastened to a second vane of said magnetron, said first vane beingpositioned adjacent to said second vane, and the position of fasteningon said second vane being close to the end near the center or saidmagnetron of said second vane.

JOHN E. EVANS. ROBERT C. FLETCHER. FOSTER F. RIEKE.

ltciicrcnces Cited in the file of this patent UNITED STATES PATENTSNumber I Name Date ii,4ll9,ti4ll Moles Oct. 22, 1946 2,417,?89 dpencer MMar. 18,. 1947 2,48l,l5i Powers Sept. 6, 1949 2,520,955 Ell-tress Sept.5, 1950 2,530,172 Oirress Nov. 14, 1950

